The current-source converter, typically a three phase thyristor bridge, has been known for many years. This type of bridge has been implemented with thyristor devices because these devices have been the only existing semiconductor switching devices having the necessary voltage and current capabilities for most of the bridge applications. Initially used for DC motor drives and for general purpose DC power supplies, this type of converter has also been used for high voltage DC transmission (HVDC). More recently, it has been used for AC motor drives.
In all these applications, the thyristor bridge has to rely on the AC line voltage for commutation, i.e. "natural" commutation. As a result, it exerts a lagging quadrature current demand (operates at a lagging power factor) on its AC supply, and demand varies with both the DC current level and the DC voltage-to-AC voltage ratio. Moreover, in its basic form, the thyristor converter produces large harmonic current components into the AC source.
Despite these problems, this type of converter has received wide acceptance because it is cheap, efficient, reliable, and tolerant of fault conditions as well as being able to survive many such fault conditions. Such advantages are due to the rugged nature and the low cost of thyristors. The defects, though, stem from the switching characteristics of those devices. It has long been recognized that forced-commutation could improve the power factor behavior and that by increasing the switching rate, one can so modify the production of harmonics that smaller filters are called for. However, with thyristors as the main switching devices, forced-commutation necessitates the addition of forced-commutating circuits. These are complex, expensive, inefficient and they seriously degrade the reliability of the converter. This is the reason why forced-commutated ("self-commutated") converters have not been used extensively.
In order to overcome this, in accordance with the present invention, a solution is proposed taking advantage of the coming into existence of gate-turn-off thyristors (GTO) which allow the implementation of forced-commutation without the need for commutating circuits. The invention involves the use of GTO's in a current-source converter. The operational advantages of good power factor and better harmonic behavior are therefore brought about. Besides this, the main goal is, in the field of power converters to take advantage of the similarities between the GTO and the thyristor in implementing, with GTO's, fault survival strategies such as have been used with thyristors.